This invention relates to a process for the extraction and recovery of metals from ores, particularly low grade ores, by grinding the ores to an extremely small particle size, forming a substantially homogeneous aqueous slurry of the particles, and extracting metals from the slurry with a solvent in a counter-current liquid-slurry extraction step.
In the past, many processes for the extraction and recovery of precious metals, such as gold and silver, from ores by hydrometallurgical methods have been well known. The present invention is an improvement on the cyanide process, although in principle any liquid material which acts as a solvent for precious metals may be used. The cyanide process has been known since the late 19th century. In this process, finely divided ore is treated with a dilute solution of potassium or sodium cyanide, air, and a base (for pH control). Gold, silver, and other metals, such as copper, react with the cyanide solution in the presence of oxygen and are dissolved. The chemistry of this process, known as the MacArthur process, is as follows: EQU 4NaCN+2Au+1/2O.sub.2 +H.sub.2 O.fwdarw.2NaAU(CN).sub.2 +2NaOH
Recovery of the dissolved metals is effected by treating the solution with elemental zinc or aluminum dust.
The cyanide process has not been economical for use in low grade ores, and the reaction rate of dissolution of the gold from the ore is so slow that the process must be carried out on a batch basis. Indeed, the process is typically operated by dumping raw materials into a vessel, and leaching the materials with a cyanide solution for a period ranging from one or two days to weeks and even months before even modest levels of recovery were achieved. For many ores, a 50% recovery of gold was considered quite successful.
Because of its limited product recovery and time-consuming nature, this process has proven commercially useful only for rich or fairly good grade ores. In addition, in processes wherein the cyanide solution was percolated either upwardly or downwardly through crushed ore, the portion of the ore having a particle size below about 3 mm was rejected to facilitate the movement of the solution through the rock; as a result, losses as high as 30% of the metal were attributed to these unprocessed fines. Because of the slow reaction rate, in addition to sedimentation and abrasion problems caused by the handling of large size particles, processes of the prior art have always been operated on a batch basis. No liquid extraction process for removal of trace quantities of metals from ores has been successfully operated on a continuous basis.
According to the present invention, a process for the continuous extraction of metals from ores is provided in which the ore is first ground or pulverized to a submicron size, a substantially homogeneous aqueous slurry of the pulverized particles is formed, and the slurry is contacted with a liquid solvent for the metals to be recovered in a counter-current centrifugal extraction step. It has been found that when the metal containing ore is ground to submicron size, the dissolution of metal when contacted by solvent is substantially instantaneous. By way of comparison, in the normal leaching process wherein a pile of crushed ore is leached by contact with cyanide solution for one month to provide a recovery of less than 50%, the speed of reaction when contacting a submicron size particle of the same rock is less than 1/2 second. In addition, it has been found that because an aqueous slurry of the submicron size particles can be handled and pumped as a homogeneous liquid, the slurry can be treated in a liquid-liquid extraction process in the same manner as an extraction process involving two immiscible liquids are processed. This sequence of process steps enables an entirely continuous process operation, permitting very good quality control of processing parameters, and resulting in an extraction of in excess of 90% of the metal content of the ore. As a result, very low grade ores which are normally discarded as tailings or scrap can be economically processed according to the invention.
Accordingly, it is an object of the present invention to provide a continuous process for the extraction of metals from low grade ores. It is a further object of the invention to provide a process wherein the ore is ground to a sufficiently small particle size to permit handling of slurries of the ore as liquids. It is yet a further object of the invention to provide a simple process for the extraction of metals in a substantially homogeneous slurry by contacting the slurry with an aqueous solvent for the metals to be extracted, and for separating a liquid extract and a solid waste product therefrom in conventional equipment. These and other objects of the invention will be apparent from the following detailed description of a preferred embodiment thereof.